1. Field of the Invention
This invention relates generally to heterojunction transistors and particularly to a symmetrical double heterojunction phototransistor.
2. Description of the Prior Art
Heterostructure transistors have been studied in considerable detail since they were originally proposed by Shockley in 1951 (U.S. Pat. No. 2,569,347), because the wide bandgap emitter opened possibilities of obtaining high injection efficiencies into a narrow bandgap base. Many material systems, including Si-Ge, Si-GeSi, GaAs-Ge, ZnSe-GaAs, ZnSe-Ge, and Ga.sub.1-x Al.sub.x As-GaAs, have been investigated and used to make heterojunction devices with varying amounts of success. For example, the devices described in U.S. Pat. No. 3,211,970 will have low breakdown voltages, for reasonable base thicknesses, because the carrier concentration is less in the base than in the emitter or collector. With many of the material systems investigated, defect currents through interface recombination states limit current gains to small values. At the present time, the Ga.sub.1-x Al.sub.x As-GaAs system appears most promising because junctions can be fabricated with a low density of interfacial states because of the good lattice match. U.S. Pat. No. 3,780,359 describes an (n-Ga.sub.1-x Al.sub.x As) - (p-GaAs) - (n-GaAs), 0.3&lt;x&lt;0.9, heterostructure designed for high frequency use and having a DC current gain of 25 . A further study of the same system reported in Journal of Applied Physics 46, pp. 2120-2124, May 1975, used a thinner base and achieved current gains of 350.
Interest in heterojunction devices is further increased as they are attractive for use as photodetectors because, unlike homojunction devices, materials can be chosen that are essentially transparent to the light of interest. This permits higher efficiencies as all incident photons can be absorbed where desired within the device. Devices of this type, i.e., transistors with optically accessed bases, have been made by Alferov, Soviet Physics-Semiconductors 7, pp. 780-782, December 1973, and Beneking, Electronics Letters 12, pp. 395-396, Aug. 5, 1976, who reported gains of 350 and 2,000, respectively.
There are many situations where it is desirable to provide electrical isolation between two electrical circuits. For example, it may be desired to insure circuits are independent of each other with respect to DC potential levels or to restrict spurious or unwanted voltages to a single part of the complete electrical system. Many devices, including relays and transformers, can provide electrical isolation. However, for some purposes, primarily relating to size, it has been found desirable to couple circuits optically.
Devices providing such optical coupling are commonly called opto-isolators. These are solid state devices consisting of a light source connected to the input circuit, commonly a light emitting diode (LED), and a photodetector connected to the output circuit. A current flowing in the input circuit causes the light source to emit and some of the light falls on the photodetector which causes the output circuit to respond. Because of their small sizes, opto-isolators are well-suited for use in solid state equipment.
Although photodetectors of the type described can be used in opto-isolators, they suffer a drawback that complicates their use in opto-isolators. For many purposes, it is more convenient to operate the photodetector with either polarity of applied voltage. The devices described will work with only one polarity of voltage across the photodetector and care must therefore be taken to insure that the voltage polarity is always correct. In many cases, such as supervisory signaling in telephone systems, this is inconvenient or impossible and a bilateral switch is desired.